How can i make Blender export multiple meshes with PLY/STL format for printing in 3D?
You can (maybe on a file copy, or even temporary just to export) convert the posed mesh to "mesh": select the posed mesh in object mode and press ALTC, select "convert to mesh". Or, you can apply the armature modifier, is the same.

Then the posed mesh is permanent, still parented in the same way but no more deformed, so you can join it to other objects, and then export.

However, joined object may pose some problem to 3d printing, since they were not modeled as a single mesh: they will not be connected, for sure, and may have overlaps, etc. You may need to edit the joined shape to fix any 3d printing issue.

In some case 3d printing software can address lesser problems, but you can also check with the (bundled, but disabled by default) addon "3d printing toolbox" to find and address most of them.